Match shell squircle outlines with compositor masks

Use shared corner geometry for shell controls, window clipping, materials and blur masks. Preserve circular pills and controls where appropriate, track contour changes in damage, and use tagged ferese-shape and libcosmic dependencies.

docs/configuration.md

@@ -193,7 +193,7 @@ Live windows, overview thumbnails and retained images share a window identity
 and presentation state. Entering overview inherits the window's visible position
 and velocity, including motion from a workspace slide. Resize images use the same
 content mapping in overview and remain visible if the window closes during a resize.
-Content, clipping and borders use the same rounded rectangle at the output's scale.
+Content, clipping and borders use the same squircle outline at the output's scale.
 
 ### Spring tuning
 
@@ -837,7 +837,10 @@ Text input is available by default; Ctrl+Alt+Escape releases an active shortcut
 inhibitor. Use `feresectl --help` for runtime control commands.
 
 Shell rounding is controlled in Settings → Appearance → Shell corner radius.
-Small controls cap the radius to fit their size. Window rounding remains under
+The shell and compositor use the same squircle profile for fills, borders, shadows
+and blur masks. Small controls cap the radius to fit their size; circles and pills
+keep circular outlines. The measured profile has small tangent and curvature
+discontinuities at its internal segment joins. Window rounding remains under
 Settings → Windows. `theme.geometry.shell-radius` takes precedence over the old
 `appearance.corner-radius` and then `theme.geometry.top-bar-radius` keys; when
 none are set, the shell uses 14 px. Legacy clock/note radius fields no longer